1. Field of the Invention
The present invention relates to an ultra wide angle imaging optical system having an ultra wide angle of e.g. more than 180 degrees as an entire viewing angle in one direction, an ultra wide angle imaging lens device provided with the ultra wide angle imaging optical system, and an image sensing apparatus mounted with the ultra wide angle imaging lens device, such as a car-mounted camera or a security camera.
2. Description of the Related Art
Generally, it is preferable to use an imaging optical system having a wide viewing angle i.e. a wide angle lens in a car-mounted camera to be mounted as a back monitor in an automobile, or in a security camera for monitoring a predetermined monitoring area, or in a like device so as to acquire video information concerning a wider area with a less number of cameras. In the car-mounted camera, however, a wide viewing angle is not required in all directions, but is primarily required in a horizontal direction i.e. a widthwise direction of the automobile. This is because generally a wide area observation is not required in a vertical direction i.e. a height direction of the automobile, and because the car-mounted camera with a wide viewing angle in the vertical direction may capture an image of a high luminance object such as the sun or a street lamp, which may degrade the quality of the monitored image. In car-mounted cameras or security cameras of a recent trend, there is a demand for securing an ultra wide angle of more than 180 degrees, as the entire viewing angle in the horizontal direction.
In the case where a normal fisheye lens arrangement is adopted to realize such an ultra wide angle imaging optical system, the viewing angle in the vertical direction may be unduly increased. As a result, an image distortion resulting from lens distortion may be increased, in addition to the likelihood that a high luminance object image may captured. In view of this, there is proposed an anamorphic optical system having focal lengths or lens distortions different from each other in the horizontal direction as a first direction and in the vertical direction as a second direction. Use of the anamorphic optical system enables to provide an imaging optical system capable of securing an ultra wide angle in the horizontal direction where a wide viewing angle is required, and yet with a relatively less image distortion in the vertical direction. Such an anamorphic optical system is disclosed in e.g. Japanese Patent No. 2,984,954 and Japanese Unexamined Patent Publication No. Hei 8-62494. The publications disclose an arrangement in the field of electronic endoscopes, in which an anamorphic surface is provided in an objective optical system to match the viewing angle of a lens element of the objective optical system with an aspect ratio of a display screen.
The anamorphic optical system disclosed in the publications is constructed in such a manner that a forward lens group as a first lens group, an aperture stop, and a rearward lens group as a second lens group are arranged in order from the object side, and that merely a single negative lens element is provided on the object side of the first lens group. In this arrangement, the production of an ultra wide angle imaging optical system having a viewing angle of more than 180 degrees in a horizontal direction is difficult. Also, since the anamorphic surface is provided on the first lens element having a large lens diameter, the cost relating to formation and processing of the anamorphic surface may be increased.